linux-kernel - Re: Assignment of GDT entries

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Message-ID: <787b0d920609140012i220a189es68d077f3c67c68e2@mail.gmail.com>
Date:	Thu, 14 Sep 2006 03:12:14 -0400
From:	"Albert Cahalan" <acahalan@...il.com>
To:	"Zachary Amsden" <zach@...are.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>, torvalds@...l.org,
	jeremy@...p.org, mingo@...e.hu, ak@...e.de, arjan@...radead.org,
	linux-kernel@...r.kernel.org
Subject: Re: Assignment of GDT entries

On 9/14/06, Zachary Amsden <zach@...are.com> wrote:
> Albert Cahalan wrote:

> > So basically it's not allowed to just grab the 3rd slot?
>
> You can, but you should be prepared for it to fail as well.

Without knowing details of the kernel's GDT, how?

> > What if I want to find out what is already in use?
> > Am I supposed to iterate over all 8191 possible
> > GDT entries? How do I even tell how many slots
> > are available without using them all up?
>
> There are only 32 possible GDT entries in 32-bit i386 Linux, and only
> three of them are usable for userspace.  You can't find out which slots
> are in use, but you can cause one to be allocated and returned to you.
> This seems like a perfectly reasonable API to me, why do you think it is
> so ugly?

Eh, "returned to you" doesn't work for me. I need to
figure out what other code (not written by me) uses.

I may need to "borrow" a slot if all three slots are in
use. Without using evil knowledge of the GDT, how
am I to do that? I don't know what slots might have
been allocated by other libraries.
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